Perl Functions Learning Objectives: 1. To learn how to create functions in a Perl’s program & how to call them 2. To learn how to pass [structured] arguments into functions & return value from a function 3. To understand the scope of a Perl’s program

COMP111 Lecture 18 / Slide 2 Perl Functions Table of Content  Defining a Functions  Calling a Function  Returning Values  Arguments  Local Variables  Keywords: use strict  More on Parameters  Structures as Parameters  Recursion – Recursive processing of directory tree

COMP111 Lecture 18 / Slide 3  A user-defined function or subroutine is defined in Perl as follows: sub subname{ statement1; statement2; statement3; }  Simple Example: sub hello{ print "hello world!\n"; } Defining a Function (1)

COMP111 Lecture 18 / Slide 4 Defining a Function (2)  Subroutine definitions can be anywhere in your program text (they are skipped on execution), but it is most common to put them at the end of the file.  Within the subroutine body, you may use any variable from the main program (variables in Perl are global by default). #!/usr/local/bin/perl5 -w $user = "horner"; hello(); print "goodbye $user!\n"; sub hello{ print "hello $user!\n"; }

COMP111 Lecture 18 / Slide 5 Calling a Function  You can also use variables from the subroutine back in the main program (it is the same global variable): $ cat sum1 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $sum = 0; sum_a_and_b(); print "sum of $a plus $b: $sum\n"; sub sum_a_and_b{ $sum = $a + $b; } $ sum1 sum of 1 plus 2: 3 $

COMP111 Lecture 18 / Slide 6 Returning Values (1)  You can return a value from a function, and use it any expression as in C++: $ cat sum2 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $c = sum_a_and_b() + 1; print "value of c: $c\n"; sub sum_a_and_b{ return $a + $b; } $ sum2 value of c: 4 $

COMP111 Lecture 18 / Slide 7 Returning Values (2)  A subroutine can also return a list of values: $ cat list1 #!/usr/local/bin/perl5 -w $a = 1; $b = = list_of_a_and_b(); print "list of sub list_of_a_and_b{ return ($a,$b); } $ list1 list of c: 1 2 $

COMP111 Lecture 18 / Slide 8 Returning Values (3)  Another example: $ cat max1 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $max = max_of_a_and_b(); print "max: $max\n"; sub max_of_a_and_b{ if($a > $b){ return $a; } else{ return $b; } } $ max1 max: 2 $

COMP111 Lecture 18 / Slide 9 Arguments (1)  You can also pass arguments to a subroutine.  The arguments are assigned to a list in a special for the duration of the subroutine. $ cat max2 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $max = max($a, $b); print "max: $max\n"; sub max{ if($_[0] > $_[1]){ return $_[0]; } else{ return $_[1]; } } $ max2 max: 2

COMP111 Lecture 18 / Slide 10 Arguments (2)  A more general way to write max() with no limit on the number of arguments: $ cat max3 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $max = max($a, $b, 5); print "max: $max\n"; sub max{ $max = 0; foreach $n if($n > $max){ $max = $n; } } return $max; } $ max3 max: 5

COMP111 Lecture 18 / Slide 11 Arguments (3)  Don ’ t confuse $_ they are unrelated.  Excess parameters are ignored if you don ’ t use them.  Insufficient parameters simply return undef if you look beyond the end of array. is local to the subroutine.

COMP111 Lecture 18 / Slide 12 Local Variables (1)  You can create local versions of scalar, array and hash variables with the my() operator. $ cat max4 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $max = 0; $max1 = max($a, $b, 5); print "max1: $max1\n"; print "max : $max\n"; sub max{ my($max,$n);# local variables $max = 0; foreach $n if($n > $max){ $max = $n; } } return $max; } $ max4 max1: 5 max : 0

COMP111 Lecture 18 / Slide 13 Local Variables (2)  You can initialize local variables: $ cat max4 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $max = 0; $max1 = max($a, $b, 5); print "max1: $max1\n"; print "max : $max\n"; sub max{ my($max,$n) = (0,0); # initialized locals foreach $n if($n > $max){ $max = $n; } } return $max; } $ max4 max1: 5 max : 0

COMP111 Lecture 18 / Slide 14 Local Variables (3)  You can also load local variables directly : $ cat max5 #!/usr/local/bin/perl5 -w $a = 1; $b = 2; $max = max($a, $b); print "max: $max\n"; sub max{ my($n1, $n2) if($n1 > $n2){ return $n1; } else{ return $n2; } } $ max5 max: 2

COMP111 Lecture 18 / Slide 15  You can force all variables to require declaration with my() by starting your program with: use strict; $ cat max5 #!/usr/local/bin/perl5 -w use strict; my $a = 1;# declare and initialize $a my $b = 2;# declare and initialize $b my $max = max($a, $b); # declare and initialize print "max: $max\n"; sub max{ my($n1, $n2) # declare locals if($n1 > $n2){ return $n1; } else{ return $n2; } } $ max5 max: 2 Keyword: use strict (1)

COMP111 Lecture 18 / Slide 16  use strict effectively makes all variables local.  Typing mistakes are easier to catch with use strict, because you can no longer accidentally reference $billl instead of $bill.  Programs also run a bit faster with use strict.  For these reasons, many Perl programmers automatically begin every Perl program with use strict.  It is up to you which style you prefer. Keyword: use strict (2)

COMP111 Lecture 18 / Slide 17 More on Parameters  Variables are passed by name in Perl, i.e., parameters from the caller can be changed. $ cat swap #!/usr/bin/perl -w sub swap { ($_[0],$_[1])=($_[1],$_[0]); } ($a,$b)=(1,2); swap($a, $b); print "$a, $b print \n"; $ swap 2, Arrays are expanded like lists when they are in parameters will be (1, 2, 1, 2) instead of two arrays. Arrays are expanded like lists when they are in parameters will be (1, 2, 1, 2) instead of two arrays.

COMP111 Lecture 18 / Slide 18 Structures as Parameters  To pass arrays, hashes or other structures, use pointers: $ cat vsum #!/usr/bin/perl -w sub vsum{ return undef if for ($i=0; $i $i++ ) { $sum[$i] = $_[0][$i] + $_[1][$i]; } print print $ vsum Sum1: 4 6 Sum2: 6 first second } gives element count in a scalar first second } gives element count in a scalar context $_[0][$i] i-th element (cf, 2D array) [ ] returns reference of an anonymous array We cannot use ( ) here [ ] returns reference of an anonymous array We cannot use ( ) here

COMP111 Lecture 18 / Slide 19  Here is the skeleton of a program for recursive directory processing: #!/usr/local/bin/perl5 -w sub dirtree { = ; # local variable required print "Directory listing for: $_[0]\n"; foreach print "$_\n"; dirtree($_) if (-d $_); # recursion } dirtree($ARGV[0]); Recursive Directory Processing